Iron (a chemical element with the symbol of Fe) is a mineral that is present as a natural component of many foods. Within the body, iron is a necessary part of hemoglobin, which moves oxygen from the lungs to the tissues, and myoglobin, a protein that supplies oxygen to the muscles as a function of metabolism. Further, iron is a necessary component for the proper development and functioning of the body's cells.
Iron is lost in the body during exercise for a number of reasons including possible cuts and scrapes that result in bleeding possible loss of blood in the gastrointestinal system, through sweating, foot strike hemolysis (from running on hard surfaces), and increased turnover of red blood cells during exercise.
The mineral iron is essential to the maintenance of good health and normal body functioning. About one milligram of iron, however, is lost per day in the average human. For athletes, the daily amount of recommended dietary iron is 1.3 to 1.7 times higher than for nonathletes. Thus, as iron is lost from the body, an equivalent amount must be absorbed into it, which can only be accomplished with diet. That is, the body can only acquire iron by absorbing it from food and drink, which means that iron is considered an essential nutrient (one that must be acquired from an outside source).
A proper amount of iron is essential for a number of functions in the human body, perhaps the most important of which is the transport of oxygen from the lungs to cells. Iron is a constituent of a complex called heme, the oxygen-carrying component of the molecule hemoglobin, found in all red blood cells. Red blood cells transport oxygen when oxygen molecules bond loosely to iron atoms in heme and are transported throughout the bloodstream.
Iron also occurs in the body in the form of a protein complex called ferritin. Ferritin is stored in body tissues, especially the liver, spleen, and bone marrow, from which it can be released for use in the formation of heme and in other bodily functions.
A deficiency of red blood cells, or hemoglobin, results in a condition known as iron-deficiency anemia, but a more general deficiency of iron in the body, which might include a deficiency of ferritin, results in a condition known as iron deficiency. The two conditions may or may not be related with each other, and separate tests for each are available.
According to the World Health Organization (WHO), iron deficiency is one of the most common nutrient deficiencies found in the world. From 40% to 80% of women in developing countries have iron deficiency. Overall, approximately 16% of the world's population is thought to be deficient in the element. In the United States, about 16% of adult woman have iron deficiency but are not anemic.
Normally, the body of an average adult male contains about 4 grams of iron, whereas an average adult female has about 3.5 grams. Because of its miniscule, but important, presence in the body, iron is called a trace element, with only about 0.005% of the human body being made of iron. Although small in concentration, iron is still found throughout the body, within such places as blood proteins, bone marrow, enzymes, muscles, and tissues.
The issue of iron loss during exercise is of special concern to anyone who engages in active exercise, especially endurance events, such as long-distance runs, cycling, or swimming. Iron loss is of much more concern to females than to males because females lose iron via menstrual blood.
Medical researchers have been studying exercise-related iron loss for well over a century. The condition was first described in 1881 by the German researcher R. Fleischer. From a purely logical standpoint, it would seem that iron loss would be an inevitable consequence of exercise, especially exercise that continues for extended periods of time. Medical researchers now have little doubt that exercise results in the loss of red blood cells and hemoglobin in a variety of ways.
Another mechanism of blood loss was discovered, one that occurs with middle- and long-distance runners. The longer a person runs and the more their feet pound on hard surfaces, the greater the opportunity that pressure on the soles of the feet will “squash” and destroy red blood cells in the soles of the feet, providing yet another means by which the body loses iron. This medical condition is called foot strike hemolysis.
In 2016, the preponderance of the research evidence available indicated that exercise is correlated with iron-deficiency anemia, especially in young female athletes. Loss of iron through perspiration (higher sweat losses results in higher iron losses), gastrointestinal bleeding (minor damage to the stomach and intestinal lining results from strenuous exercise), cuts and wounds (as blood is expelled from the body), and foot impact (in the condition called foot strike hemolysis) have all been implicated in exercise-related anemia. Other factors may also be involved in low red blood cell counts observed in athletes. Among females, menstrual loss is a significant factor.
Even more important in many instances is dietary choice, in that both male and female athletes may consume diets low in iron content, placing them at risk for exercise-related anemia. Vegetarians and vegans are at special risk for low serum iron levels because they do not eat meat, which is among the best natural sources of iron. For this reason, dietary iron recommendations for vegetarians are 1.8 times higher than for meat eaters.
Exercise-related anemia has also been found within many research studies to be correlated with reduced performance. For instance, researchers at Cornell University in Ithaca, New York, reported on a review of 29 studies of humans and other animals with severe-to-moderate iron-deficiency anemia. They found a clear correlation between the level of performance of both humans and other animals and the degree of anemia observed for the subjects of these studies. They concluded that a deficiency in red blood cells caused a reduced ability to carry out normal respiratory functions, and suggested that iron supplementation may be advisable in conditions in which iron-deficiency anemia was greatest.
Research shows commonality of results with respect to iron status and exercise. The paper “Iron Status and the Acute Post-Exercise Hepcidin Response in Athletes,” published in March 2014 in the journal PLOS ONE, summarizes these research results, stating that “applied sports physicians, dietitians, and physiologists working with iron deficient athletes should continue to focus on increasing iron stores through food choices and oral supplementation; or via intramuscular and intravenous iron injections.”
Hepcidin, an iron regulatory hormone produced by the liver, regulates iron's entry into the circulatory system. Further, hepcidin is affected by nutritional patterns, specific exercise activities, and training frequency. Studies have shown that elevated hepcidin levels after exercise may play a major role in the large numbers of athletes who are diagnosed with iron deficiency.
Exercise-related anemia is an understandable and expected phenomenon because of the stresses placed on the body by physical exertion. Sweating, gastrointestinal bleeding, internal hemorrhaging of blood cells, foot impact, and other events associated with physical activity all result in the loss of red blood cells and, hence, cause iron-deficiency anemia.
A number of factors unrelated to exercise are also significant contributors to exercise-related anemia, including loss of menstrual blood among females and poor dietary choices by both males and females that result in deficient iron stores regardless of their participation in exercise and athletic events.
One consequence of research findings is that trainers and athletes may benefit from determining an individual's red blood count prior to participation in an exercise. A person with a low red blood count may benefit from iron supplementation either by following an improved diet or by taking iron supplement pills. One caution to be observed is that low red-blood-cell counts may not represent a true iron-deficiency anemia. Researchers have long been aware of a type of anemia known as sport anemia, or dilutional pseudoanemia, a condition in which an athlete's red-blood-cell count is lower than normal during or following an exercise. The condition is especially common among endurance athletes and does not represent a medical problem.
Dilutional pseudoanemia occurs naturally when a person exercises for an extended period of time and the body increases the total blood plasma volume by a significant amount, sometimes as much as 20%. In such a case, a measure of a person's red blood cell count, in cells per milliliter, is lower than might be expected. It is not low because of a reduction in red blood cells, but because of the increased volume of blood plasma present in the body.
Athletes who are concerned about possible irondeficiency anemia problems should consult with their sports physician or trainer about having regular blood tests to determine the level of iron in their blood, with the possibility of taking an iron supplement. The recommended supplemental dosage for iron is 100 milligrams (mg) per day taken on an empty stomach. Iron supplements should never be taken without the advice and supervision of a physician.
The following groups, especially if the individuals are athletes or if they exercise frequently or strenuously, are at increased risk of iron deficiency anemia.
Risks are associated with both too small and too great an intake of supplemental iron. A person should make every effort to obtain the recommended daily intake of iron through foods and resort to supplements only with the advice and supervision of a physician. Excess intake of iron can cause a number of immediate and short-term problems, such as nausea, fatigue, and disorientation, as well as a number of serious long-term consequences, such as heart and brain damage.
The inadequate intake of iron, or iron deficiency, is associated with anemia, along with angular stomatitis (inflammation of the mucosa of the mouth), blue sclera (discoloration, often with a blue-gray color, of the whites of the eyes), esophageal webs (also called Plummer-Vinson syndrome; webs that develop in the upper esophagus, which make for difficulty in swallowing), and glossitis (inflammation of the tongue).
Early signs of iron deficiency are irritability and lethargy (feeling tired). A feeling of not wanting to exercise may accompany those early warning signs. Over time, long-term symptoms include:
Iron supplementation can improve both a person's general health and athletic performance, especially if a person is iron deficient before undertaking the exercise. Meeting the recommended daily intake of the mineral through foods (e.g., red meat, dark leafy greens, beans, prunes, etc.) is always a preferred approach, but athletes who are likely to experience exercise-related anemia, such as endurance runners and cyclists, may want to consider taking iron supplements to deal with the anemia. Evidence suggests that the effect of supplementation is related to anemia severity: the more severe the anemia, the more useful the supplementation. Individuals with mild iron-deficiency anemia gain little or no benefit from iron supplementation.
Without iron supplementation, athletes and others who exercise regularly should eat foods rich in iron. Two types of iron exist in foods: heme iron and nonheme iron. Heme iron is found in the muscle of meat such as beef seafood, pork, and poultry. Fifteen to 18% of heme iron is absorbed into the body. Of note, lean meat such as beef contains about three times more iron than chicken or fish.
Nonheme iron is found in plant-derived foods. These sources include cereal grains and fortified cereals, eggs, legumes, some green vegetables (especially lentils and leafy vegetables), dried fruits, and nuts. About 5% less (10% to 13%) nonheme iron is absorbed into the body, compared to heme iron absorption.
In both cases, avoid drinking caffeinated beverages while eating iron-rich foods because caffeine can impair the absorption of iron. The chemical compounds called phytates and oxalates, found in some grains, such as wheat bran, and some vegetables, such as spinach and rhubarb, reduce the absorption of iron. In addition, calcium-rich beverages, such as milk, should not be eaten with iron-rich foods because calcium can reduce the absorption of iron into the body.
See also Nutritional supplements .
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David E. Newton, AB, MA EdD
Revised by William A. Atkins, BB, BS, MBA